New Species and New Records of Podostemaceae from Phitsanulok Province, Northern Thailand

Podostemaceae comprise a unique aquatic family of submerged angiosperms in open, sunny habitats in torrential water of tropical and subtropical rivers in seasonal climates, and attached to rocks or boulders in waterfalls and river-rapids (Cook 1996, Cook & Rutishauser 2007). The plants emerge above the water to produce flowers and fruits only in the dry season (Willis 1902, Kato 2013). Wherever water levels are seasonally unchanged in natural or artificial conditions, the plants are incapable of completing their life cycle. Due to this special habitat, Podostemaceae have been poorly collected compared with terrestrial plants. Knowledge of their significance in flora has therefore been limited. The family Podostemaceae comprises approximately 54 genera and 300 species in three subfamilies (Cook & Rutishauser 2007, Koi et al. 2012). Subfamily Podostemoideae, the most diverse (ca. 47 genera and ca. 280 species), is distributed worldwide, Tristichoideae (6 genera and ca. 19 species) is mainly distributed in Asia, with exception of Tristicha trifaria (Bory ex Willd.) Spreng., which is widely dispersed from Central and South America to Africa, and Weddellinoideae (a monotypic genus), which is confined to South America (Kato 2006a). The three subfamilies can be separated by floral features. PodosteActa Phytotax. Geobot. 67 (2): 97–114 (2016)